The incidence of hypothyroidism was analyzed in 4,553 hyperthyroid patients treated with I-131 between 1951 and 1975. The average annual rate of hypothyroidism during the first 7 yr after therapy increased continuously for each 5-yr period of treatment, from 3.6% for patients treated between 1951 and 1955 to 7.7% for patients treated during the period from 1971 though 1975 (p less than 0.001). The increase in the incidence of hypothyroidism was seen before the introduction of TSH assays and when allowance was made for thyroid gland size, the age of the patient, and the experience of the radiotherapist. The use of TSH assays has probably resulted in an earlier recognition of hypothyroidism, which may explain why the most marked rise in the incidence of hypothyroidism was observed after the introduction of TSH assays.